The properties of poly(ADP-ribose) polymerase associated to nucleosomes and nuclei were analyzed by measuring enzymatic activity, isolating poly(ADP-ribosylated) histones and by electron microscopy. The enzyme was found to be most active in tri- and tetranucleosomes. The presence of NaC1 (0 to 200 mM) in the incubation medium showed a very small inhibitory effect on the enzymatic activity of isolated nucleosomes; however, the enzyme activity of incubated nuclei showed a gradual reduction with increasing salt concentrations leading to an almost complete inhibition at 200 mM NaC1. In parallel, the histones H1 extracted from isolated nucleosomes incubated at 200 mM NaC1 were hyper(ADP-ribosylated), whereas the histones H1 extracted from nuclei chromatin showed no highly modified forms at 200 mM NaC1. The effect of ethidium bromide (EB) was also analyzed and was found to stimulate enzyme activity in tetranucleosomes and octanucleosomes but not in dinucleosomes and nucleosomes of 20 repeat units. Electron microscopic analysis of poly(ADP-ribosylated) nucleosomes does not show the presence of aggregates at low ionic strength. However at higher ionic strengths the presence of localized relaxed domains of chromatin is evidenced in condensed nucleosomes.